1. Field of the Invention
This invention relates to the field of tapered threaded pipe joints. It particularly concerns a method and apparatus for use in the gaging and joining of tapered, threaded pins and boxes.
2. Description of Prior Art
Pipe sections used in oil fields (for example, long sections of well casing or tubing) usually have tapered, exteriorly-threaded male ends, called "pins." Such ends are threaded into collars (which are short female pipe sections the threaded portions of which are often called "boxes"), there being an interiorly-threaded tapered box region at each end of each collar. The tapered, threaded joints are very difficult to make up (form) properly.
For example, because the male and female threaded regions are tapered (frustoconical), there can only be a certain amount of penetration of the pin into the box before permanent deformation of the threads occurs. Such permanent deformation is not desired for reasons including the fact that the pins and boxes are then not reusable. In extreme cases, the box may split. Conversely, however, there must be a sufficient penetration to achieve good sealing against gas or oil leakage, to achieve adequate resistance to axial tensile stresses, etc.
There are two major factors that determine whether or not a joint between two tapered pipe regions (namely, between a pin and a box) is correctly made. The first factor is the degree of penetration, or amount of engagement, as stated above. The second factor is the makeup torque, namely the torque which exists at the very last increment of the makeup procedure. Makeup torque values have been established by the American Petroleum Institute ("API") for each size, weight, and grade of casing and tubing.
Although it has long been known that there must be both (a) proper engagement (namely, proper degree of penetration) and (b) proper torque, in order to have a correctly made up tapered joint, workers in oil fields do not now achieve these factors conjointly with any degree of regularity. Instead of knowing when there is proper engagement (degree of penetration) in each joint, at the proper torque, it is conventional practice to rely upon approximations, estimates and surmises. It is not conventional to gage the pin and/or box at each and every joint and then use the results of the gaging to achieve good connections. The present invention makes it practical to gage the pin and box for each joint, even in the field, and employs the results of the extensive gaging to assure that there is proper engagement (penetration) at each joint.
The gaging effected by the present apparatus and method produces, as one of its major benefits, the correct location of the actual "hand tight plane." The hand tight planes of tapered pipe and collar members are planes normal to the axes of each, which planes are coplanar when the two members are in so-called hand tight engagement. When in such hand tight engagement, the threads have been interengaged to a point where they are in intimate contact but without deformation. This normally will occur at a torque of from ten to eighty foot pounds for seven inch long thread (8-round) casing, for example.
From another point of view, the hand tight planes of the collar and pipe are coplanar when the two have been threaded together to such an extent that any further threading of the male member into the female member will commence to deform the threads. (This hand tight engagement of tapered threaded elements is analogous to an interengagement of male and female smooth-surfaced frustoconical members, which are axially interengaged to a point of intimate contact in which no significant deformation takes place. With such smooth frustoconical members, further interengagement from this analogous hand tight engagement would require deformation of the two parts, namely a decrease in diameter of the male and/or an increase in diameter of the female.)
The hand tight plane is defined by the American Petroleum Institute in the API Specification for Threading, Gaging, and Thread Inspection of Casing, Tubing, and Line Pipe Threads, API Standard 5B, Ninth Edition, March, 1974, which is hereby incorporated by reference as though fully set forth herein. Page 6 of this Specification shows (FIG. 2.1) that the plane of hand tight engagement of the collar is at a distance M from the end of the coupling, and that the plane of hand tight engagement of the pipe is at a distance L.sub.1 from the end of the pipe. These two planes are coincident in the condition of hand tight makeup of the two elements. (Reference is also made to FIG. 1 of the present patent application.)
Values of M and L.sub.1 are given in this API Specification for different sizes and thread types of casing, line pipe and tubing. It is emphasized, however, that the API data concerning M and L.sub.1 are not used by applicants to locate the hand tight plane. The reason M and L.sub.1 data are not used by applicants for hand tight plane locations is that the API permits substantial tolerances which applicants desire to eliminate.
Unless the actual (not theoretical) hand tight plane or planes are known, you can't be sure whether or not there is proper engagement (degree of penetration of the pin into the box) in the completed joint. Furthermore, as stated above, unless there is proper engagement at the proper torque you can't be assured of a correctly made up joint.
It might be thought that since the pipes (and collars) are mass produced in pipe mills, the hand tight planes thereof are the same and could be known by (for example) a mark made a certain distance from the end. This is not so, since there are manufacturing tolerances which make the pipes (and collars) far from perfectly uniform. The only practical way to be sure of locating the hand tight planes correctly is to use a standard thread gage on each pipe and collar element.
It is an important advantage of the present apparatus and method that they work properly on tubular sections (tubing and casing sections and collars) having "standard" API threads, such as those referred to in the above-cited API Specification. In most of these standard API threads, the cross-sectional shape of each thread is substantially triangular. (The word "substantially" is used because, for casing and tubing threads, the apex is often rounded--the threads being therefore conventionally termed "round.") Other API threads include buttress, etc.
The present apparatus and method not only locate, and make extensive use of, the true hand tight planes, but they also achieve other important benefits of extensive field gaging. Thus, for example, the gaging informs the user whether or not there has been any thread damage. Such damage may result from various factors existing after the pipes and collars leave the factory.
Very importantly the present gage apparatus can be employed while the pipe sections are horizontally positioned in racks near the wellhead. It is thus known, before the actual wellhead is reached, whether or not each pin and box meets API requirements. The gage apparatus can also be employed at pipe manufacturing plants, threading plants and along pipelines. It is preferred that gaging occur in the field, since changes subsequent to leaving the factory, etc., will then be detected.
Pipe gages (ring gages and plug gages) for tapered pins and boxes have, of course, long been used. They are often heavy, and thus difficult to "start" (commence threading) without false starts and/or cross-threading. There is thus a major need for plug gages and ring gages which--even when heavy and cumbersome--can be used quickly and easily. The present invention not only provides such gage apparatus but further achieves the great added benefits of simple, practical and economical pipe-marking means adapted to mark the pipes and collars in accordance with the actual locations of the hand tight planes. The marks and suitable torque gages are then used at the wellhead, in a very simple manner requiring almost no wellhead time, to conjointly achieve both (1) proper penetration (engagement) and (2) the proper, measured makeup torque.